﻿<p>A rational B-spline curve with knots is a B-spline curve
described in terms of control points and basic functions. It
describes weights in addition to the control points defined at the
supertype <em>IfcBSplineCurve</em>.</p>

<p>All weights shall be positive and the curve is given by:</p>

<blockquote><img src="../../../figures/ifcrationalbsplinecurvewithknots-math1.gif" alt="Math"border="0"></blockquote>
<p>where</p>
<table>
<tr>
<td width="100" align="right"><em>k</em>+1</td>
<td>number of control points</td>
</tr>
<tr>
<td align="right">P<sub><em>i</em></sub></td>
<td>control points</td>
</tr>
<tr>
<td align="right"><em>w<sub>i</sub></em></td>
<td>weights</td>
</tr>
<tr>
<td align="right"><em>d</em></td>
<td>degree</td>
</tr>
</table>

<blockquote class="note">
NOTE&nbsp; Entity adapted from <strong>rational_b_spline_curve</strong> in ISO 10303-42.
</blockquote>


<blockquote class="history">
HISTORY&nbsp; New entity in IFC4.
</blockquote>